Various forms of apparatus including one or more vibrating elements are well known in the field of plant or process control, one example being a contact fluid level transducer. A typical contact fluid transducer includes a pair of spaced tines mounted on a diaphragm, the diaphragm in turn defining one end of a hollow cylindrical body. The tines are arranged to vibrate at a given frequency, typically their resonant frequency, and vibration is typically effected by displacing the centre of the diaphragm by means of a compressed stack of piezoelectric elements, located within the hollow body, and driven by a cycling voltage. When the vibrating tines come into contact with a fluid, there is a change in the frequency at which they vibrate. By detecting the change in frequency, one can determine when a rising fluid level contacts the tines. Likewise one can detect when a fluid level drops below the level of the tines.
An early example of this type of level detection device is described in UK Patent No. 2 150 292. In this device a stack of piezoelectric elements is compressed against the inner or reverse side of a diaphragm by a compression screw mounted in a bridge piece extending over that end of the stack remote from the diaphragm. The spaced tines extend from the outer or front side of the diaphragm, which diaphragm forms one end of a hollow body in which the piezoelectric stack is located. The bridge piece is, in turn, mounted on a pair of spaced rod-shaped supports extending from the inner side of the diaphragm and also located within the hollow body. When the piezoelectric stack is subjected to a cyclic drive voltage, the same expands and contracts between the compression screw and the diaphragm, deforming the diaphragm and causing the tines to vibrate.
It will be appreciated that the diaphragm must be relatively robust if it is to provide a mounting point for the rod shaped supports. This requirement runs counter to a general requirement that the diaphragm be relatively thin and thus require a minimum of energy to be applied to the piezoelectric elements in order to establish vibration of the tines.
A form of apparatus which addresses this latter requirement is shown in published International Patent Application No. WO 01/95667. In this device the piezoelectric stack is compressed against the inner surface of the diaphragm by a plug. The plug has a screw thread on its periphery which engages a corresponding screw thread provided on the inner surface of the hollow body. By rotating the plug relative to the hollow body, the compression on the piezoelectric stack can be established and, if necessary, adjusted. This arrangement performs perfectly well when the instrument is operating at normal ambient temperatures, however if the apparatus is subjected to a rapid change in temperature, the wall section, to which the diaphragm and plug are attached, will react to the change in temperature to a far greater extent that the components housed within the instrument. More particularly, if the environment is subjected to a sudden rise in temperature, the wall section will expand. This will cause the compressive force applied to the piezoelectric stack to be reduced, possibly to the extent that the instrument no longer functions. If the temperature of the environment lowers rapidly, the wall section will contract. This will increase the compressive force applied to the piezo stack and may damage the crystals by crushing. Alternatively, or in addition, the lower temperature may cause the components providing the clamping force to become over-stressed and loose their ability to maintain the required clamping force when the temperature increases once again.
The problem of a device of this type, subjected to rapid temperature change, is specifically addressed in U.S. Pat. No. 6,644,116. The device described in this patent incorporates a sleeve, spaced inwardly from the outer wall, to form a mount for the compression screw or plug. In one embodiment, the sleeve is mounted on the inner surface of the diaphragm and thus has the same drawbacks as are associated with UK 2 150 292 discussed above. In a second, preferred, embodiment the diaphragm and the outer wall are provided as separate components. A collar formed about one end of the sleeve is located between the diaphragm and the wall section and the three components are then welded together. This arrangement is effective at separating the compression screw from the expanding or contracting outer wall but, because the body itself is separated from the diaphragm, the inherent integrity of the body, which is defined by the wall/diaphragm unit, is lost and the resulting device must be subjected to pressure testing to ensure fitness for purpose.
It is an object of the present invention to address the above mentioned problems; or to provide a method and/or apparatus applicable to a level detection instrument, which will at least provide a novel and useful choice.